Zika could pose more danger to unborn babies than previously thought, according to a new study which found that the virus may cause lasting eye diseases.
Scientists from University of Wisconsin-Madison in the US found that Zika virus infection passes efficiently from a pregnant monkey to its foetus, spreading inflammatory damage throughout the tissues that support the foetus and its developing nervous system.
The finding suggests that the virus poses a wider threat in human pregnancies than generally appreciated, researchers said.
Researchers followed the pregnancies from infection in the first or third trimester, regularly assessing maternal infection and foetal development and examining the extent of infection in the foetus when the pregnancies reached term.
They infected four pregnant rhesus macaque monkeys with a Zika virus dose similar to what would be transferred by a mosquito bite, and found evidence that the virus was present in each monkey's foetus.
"That is a very high level - 100 per cent exposure - of the virus to the foetus along with inflammation and tissue injury in an animal model that mirrors the infection in human pregnancies quite closely," said Ted Golos, professor at UW- Madison.
"It is sobering. If microcephaly is the tip of the iceberg for babies infected in pregnancy, the rest of the iceberg may be bigger than we have imagined," said Golos.
Three of the foetuses involved had small heads, but not quite so small relative to normal that they would meet the human standard for diagnosing microcephaly.
Microcephaly is the most striking and widely discussed result of Zika infection since Brazilian doctors raised alarm in 2014 of many babies with arrested brain development.
The new study did not find abnormal brain development, but the researchers did discover unusual inflammation in the foetal eyes, in the retinas and optic nerves, in pregnancies infected during the first trimester.
"Our eyes are basically part of our central nervous system. The optic nerve grows right out from the foetal brain during pregnancy," said Kathleen Antony, professor at UW- Madison.
"So it makes some sense to see this damage in the monkeys and in human pregnancy - problems such as chorioretinal atrophy or microphthalmia in which the whole eye or parts of the eye just don't grow to the expected size," said Antony, author of the study published in the journal PLOS Pathogens.
The similarities between the monkey pregnancies and reported complications in Zika-affected human pregnancies further establish Zika infection in monkeys as a way to study the progression of the infection and associated health problems in people, researchers said.